The instant invention is directed to a process for fabricating articles (e.g. bottles, stamped plastic sheets, rigid foams, etc.) from specific thermoplastic compositions. These compositions are either multiphase resinous copolymers having certain critical characteristics or specific ionomeric polymers (ionomers) as more fully described hereafter. This fabrication process allows production of articles which, on simple heating, change their shapes to form a predetermined new configuration. Thus, for example, a sheet or compacted slab of material prepared by this novel process can be simply heated by the ultimate user to form a container or a rigid foam without thee necessity for him to employ costly molds, etc.
It is known that crystalline and semicrystalline polyethylene and copolymers thereof with propylene may be used to prepare fabricated articles by a process in which the polyethylene is formed as a molded object, permanently cross-linked by either chemical means or by irradiation, then heated to about 140.degree.C. and collapsed and cooled. Reheating to the temperature at which the object collapsed restores the original shape. See U.S. Pat. Nos. 3,563,973 and 3,526,683. While that fabrication process has obvious advantages there are also several serious disadvantages. Both chemical and radiation crosslinking are expensive operations and increase the cost of the final product. Further, crosslinking is irreversible, i.e., once polyethylene is crosslinked, it cannot be returned to the uncrosslinked state. Therefore, it is impossible to reuse scrap. Since the process is limited to only polyethylene and certain of its copolymers, strict limitations are placed on applicability of the final product. For example, certain critical temperatures must not be exceeded, etc. By contrast, the process of the instant invention employing the thermoplastic compositions more fully described hereafter achieves all the advantages of the above-described prior art process and avoids all of the disadvantages attendant thereto.
French Patent No. 1,576,598 broadly discloses multiphase polymers which may be random, block or graft copolymers wherein either both monomers would produce a resinous homopolymer or one would produce a resinous and one an elastomeric homopolymer. For purposes of the instant invention, it has been discovered that random copolymers and copolymers wherein any monomer unit would produce an elastomeric polymer are not applicable. In fact, the only polymers applicable to the instant invention are either thermoplastic ionomers as more fully described hereafter, or graft or block copolymers falling within the following general structural formulae:
Block EQU A--B--A or (A--B).sub.n
Graft ##EQU2## wherein n is greater than 1 (i.e., two block systems are not suitable) and wherein polymer blocks A and B are each thermoplastic resins having different softening points (i.e., differing from one another by at least 10.degree., preferably by a least 20.degree., and most preferably by 50.degree.-100.degree.C.), wherein B is the lower softening block and is present in from about 50 to as high as about 97 wt. % of total polymer, more preferably about 50 to about 90 wt. %. In order that distinct phases be present in the copolymer, each polymer block should contain at least about 10 and preferably 50 or more monomeric units. It is also noted that B must be an interior block in a block copolymer or form the backbone in a graft copolymer in order to function in the instant invention. Further, the softening point of each block must be substantially above room temperature (25.degree.C.), i.e., at least about 10.degree.C. above. Therefore, the lower softening block should have a softening point of at least 35.degree.C., and should, for practical purposes, not exceed about 260.degree.C., although of course, higher softening points are also applicable. Preferably, the softening point of the lower softening block should be at least about 50.degree.C. to about 150.degree.C.
By softening point is meant either the crystalline melting point or the glass transition temperature of the polymer block.
Methods of preparing block and graft copolymers are well known and need not be recited here. In order to determine suitable monomeric constituents for a block or graft copolymer as described above, one need only determine the softening points of the homopolymers produced from these monomers and having the appropriate number of monomeric units. A blend or mixture of these homopolymers must also exist in separate distinct phases at ambient temperatures in order to insure that a copolymer prepared from selected monomeric constituents would be multiphase and useful in the instant invention.
Although these polymer blocks can best be prepared by direct polymerization techniques, these various blocks can be suitably modified by plasticization to change the softening points of the respective blocks as long as the resulting product is a multiphase product, each phase of which obeys the softening point criteria described above.
Representative examples of copolymers which are suitable for use in the instant invention provided they fall within the above-defined general formulae are: poly t-butylstyrene-polystyrene, polychlorostyrene-polystyrene, polycaprolactam-polystyrene, polycaprolactone-polystyrene, polyamides, i.e. (hexamethylene diamine-adipic acid copolymers or Nylon 6,6)-polystyrene, polypropylene-polyethylene, polybutene-1-polypropylene, polyethylene-poly 4-methylpentene-1, polymethacrylonitrile-polystyrene, polymethacrylonitrile-polyethylene oxide, polyethyleneterephthalate-polyphenylhydroxyether of bisphenol A, polyphenylhydroxyether of bisphenol A-polysulfone (from bisphenol A and dichloro diphenyl sulfone).
Preferable copolymers include: poly t-butylstyrene-polystyrene of the ABA type, crystalline polypropylene-polyethylene copolymers of the ABA type, etc.
The preparative techniques for each of these polymer blocks is well described (see Preparative Techniques of Polymer Chemistry; Sorenson and Campbell, Interscience Publishers, 1968) and the means of combining these various blocks with each other is now well known in the polymer art.
As mentioned previously, specific ionomeric polymers or ionomers are also applicable in the instant invention. These useful ionomers may be structurally defined as polymers having a backbone composed of a thermoplastic resin and having side chains or groups pendent to that backbone which groups are sufficiently polar so as to have the capability of forming ionic domains (i.e., the capability to associate with one another so as to form "physical crosslinks"). For convenience these polymers are referred to as thermoplastic ionomers.
Ionomeric polymers such as those employed in the instant invention are normally prepared by attaching acid groups to the polymer and then neutralizing the acid moiety with basic metal compounds (e.g., metal hydroxides, metal salts, etc.) or basic nitrogen compounds (i.e., ammonia, amines, etc.) to ionically link the polymers. Preferably, the metal ions employed are alkali metals or alkaline earth metals. The acid group may be introduced into the polymer chain in a variety of ways. One way is by introducing acid groups on the predominant polymer, e.g., sulfonating polystyrene. Another way is by copolymerising an alpha, beta-ethylenically unsaturated acid monomer with the predominant monomer, or by graft-polymerizing an alpha, beta-ethylenically unsaturated acid moiety on the predominant polymer.
Typical examples of ionomers employing salts of carboxylic acid type pendent groups are disclosed in British Patent No. 1,011,981; U.S. Pat. Nos. 3,264,272; 3,322,734; 3,338,734; 3,355,319; 3,522,222; and 3,522,223. Typical examples of ionomers employing phosphonate-type pendent groups include those disclosed in U.S. Pat. Nos. 3,094,144; 2,764,563, 3,097,194; and 3,255,130.
Typical examples of ionomers employing sulfonate-type pendent groups include those disclosed in U.S. Pat. Nos. 2,714,605; 3,072,618; and 3,205,285. The techniques disclosed in these references may be employed to prepare the thermoplastic ionomers of the instant invention. The thermoplastic resin used as the backbone must meet the same requirements as to softening point as the lower softening block in the previously described block and graft copolymers and any thermoplastic resin which may be suitably modified to meet these criteria is applicable. The polar groups pendent to the thermoplastic backbone should be present in from at least about 0.2 to about as high as 15 mole % (i.e., 0.2-15 moles per mole of monomer repeating unit), preferably 0.5 to 10 mole % of the total polymer.
Typical representative examples of thermoplastic ionomers useful in the instant invention include sulfonated polystyrene, sulfonated poly-tertiary butylstyrene, sulfonated polymethylstyrene, sulfonated polyethylene, sulfonated polypropylene, sulfonated polybutene-1, sulfonated styrene/methyl methacrylate copolymers, sulfonated styrene/acrylonitrile copolymers, methacrylonitrile copolymers, sulfonated polyformaldehyde and copolymers, sulfonated polyvinylchloride, sulfonated block copolymers of polyethylene oxide and polystyrene, acrylic acid copolymers with styrene, acrylic acid copolymers with methyl methacrylate. Preferably, the thermoplastic ionomer will be sulfonated polystyrene or sulfonated polyethylene and its copolymers. It should be apparent that in addition to direct sulfonation of these polymers or copolymers, a very convenient technique for incorporating a proper amount of sulfonate salt in these polymers is simply to copolymerize a suitable diene or difunctional molecule at a modest level (0.5 to 10%) with the desired monomer. For example, the copolymerization of 2 to 5 weight percent of ethylidene norbornene with ethylene using coordination catalysts provides a polyethylene with a small amount of unsaturation, yet the high crystallinity of polyethylene is still maintained. Direct sulfonation of the residual unsaturation provides a "sulfonated polyethylene" having excellent properties for the fabrication process of this invention. The same approach can be taken with nearly all of the polymers suggested above.
When the graft or block copolymers heretofore described are employed in the instant invention the process for fabricating the final article comprises the following steps. The multiphase copolymer is first heated to a temperature above the softening point of both polymer blocks. Preferably, this temperature should be at least 10.degree.C., and most preferably at least about 20.degree.C., above that of the softening point of the higher softening block. At this temperature the copolymer is in a molten state and may be readily molded to any desired form. Thus, for example, the composition may be foamed, pressed into a sheet, blow molded to form a container such as a bottle, etc. After the article has been formed in the desired shape it is then cooled to a temperature between that of the softening point of the A and B blocks and re-formed to a second desired shape. For example, if the first desired shape is a rigid foam the second desired shape may be a flat sheet. After the article is reformed in the second desired shape it is cooled to a temperature below both softening points while retaining this new desired shape. At any later time the re-formed article may be reheated to a temperature between the softening points of the A and B blocks and at this temperature will regain the first desired shape. For example, a sheet may be reheated to this temperature and expand to a rigid foam which was the first desired shape.
Another example of the applicability of this process is as follows. The thermoplastic composition is heated to a temperature above both softening points formed as a plastic sheet and cooled immediately to a temperature below both softening points. The sheet thus formed may be stored indefinitely and will retain its shape. At a later time the sheet may be reheated to a temperature between that of the softening points of the A and B blocks, stamped at that temperature with any desired design, and then cooled again to a temperature below both softening points thereby attaining a stamped plastic sheet.
One area of considerable potential for this invention relates to the use of reinforced plastics, preferably in a stamping operation. The use of reinforced plastic sheet is well-known. Glass reinforced polypropylene, containing 30 to 40% glass has been used in a stamping operation by heating the composite to a temperature above the melting point of polypropylenes, and then vacuum forming, molding, or stamping the composite into the required shape, cooling to retain dimensional stability and ejecting the part. Such a process is considered to be usable in conventional metal stamping operations. There are, however, disadvantages associated with such composites, i.e., if heated reinforced sheets are stacked at high temperature they can flow and adhere to each other. Also, the fabrication process must be conducted at a rather well-defined temperature; if too cool fabrication will not proceed; if too hot the sheet will lose its limited dimensional stability. With the compositions described in this application, the desirable handling characteristics are retained over a broader temperature range, and dimensional stability is much less temperature sensitive. Furthermore, it is within the scope of the process of this invention that a glass reinforced sheet be extruded by conventional thermoplastic means above the softening points of both phases (or in the presence of suitable plasticizers if ionic phases are present) and cooled to an intermedite temperature above the softening temperature of the lower softening phase, and simply rolled into a suitable shape for shipping (as is now done with metal sheet) followed by cooling to ambient temperature. Such a roll will be convenient to ship or store; to use, merely reheating the roll to a temperature above the lower softening phase will provide a "rubbery" sheet which can be unrolled and stamped or fabricated at will, followed by a cooling step to provide the final fabricated part.
Alternatively, this process lends itself nicely to the preparation of shrink type films often used in packaging applications. Thus, the preparation of film by means well-known in the art through extrusion techniques is easily accomplished above the softening points of both phases. Subsequently in the same operation, or at any later time, this film can be cooled (or heated) to a temperature intermediate between the softening points of the two phases, which then permits conventional mono or biaxial orientation process to be conducted. This oriented film is then cooled in the oriented state. To employ this oriented film in a packaging application, all that needs be done is to wrap the article to be packaged within the film, bond the edges in a suitable manner and then expose the film to a temperature intermediate between the two softening points. While biaxial oriented film of certain plastics such as polystyrene and polypropylene are well-known in commerce, the advantages of the two-phase systems of this invention, when conducted in the manner described are: much higher strength of the film at the intermediate temperature permitting an article to be held for very extended times at such elevated temperatures without loss of film integrity, a much descreased sensitiviity of the polymer to "melt" temperature when compared to single-phase polymers, and more general ease of application of the process to many polymer systems with a wide variety of physical properties. Numerous other uses for the instant process will be immediately apparent to one skilled in the art.
When thermoplastic ionomers are employed in the practice of the process of the instant invention, certain variations in the processing steps are required. This is necessary since most ionomers have quite high ultimate softening points (i.e., the point above which ionic domains dissociate) and, for some, the softening point is above the thermal decomposition point of the material. Therefore, preferential plasticizers, i.e., plasticizers which primarily relax ionic bonds and therefore disrupt the ionic domains of the ionomer, are employed. In order to be useful in the instant invention, these preferential plasticizers must be dispersible in the ionomer and must be liquid during processing of the ionomer. They must also possess at least one functional constituent which exhibits a bond moment whose absolute value is at least 0.6 and preferably at least 0.7 debyes. This requirement is necessary in order that there be sufficient polarity within the plasticizer molecule to attack the ionomeric cross-linkages. Typical examples of functional constituents which exhibit acceptable bond moments are listed in Table I below. This table is, of course, not meant to be exhaustivve, and any functional constituents not shown below which nonetheless have bond moments of at least 0.6 debyes are also useful.
TABLE I ______________________________________ Unit Bond Moment* (Debyes) (Absolute Value) ______________________________________ C=O (1) 2.4 C--O (1) 0.86 O--H (1) 1.53 N--H (1) 1.31 C--Cl (1) 1.56 C--F (1) 1.51 C--S (2) 1.0 C=S (2) 2.7 C--Br (1) 1.48 SH (1) 0.68 NO (3) 4.4 C=N (1) 3.6 8.fwdarw.O (4) .about. 2.9 P--Cl (1) 0.81 S--Cl (2) 0.8 Cl--O (1) 0.7 P O (4) 2.8 P--S (4) 3.2 B--O (4) 3.7 S--B (4) 3.9 ______________________________________ (1) C. P. Smythe, J. Phys. Chem., 41, 209 (1937). -(2) C. P. Smythe, J. Am. Chem. Soc., 60, 183 (1938). -(3) E. P. Linton, J. Am. Chem. Soc., 62, 1945 (1940). -(4) G. N. Phillips et al, J. Chem. Soc., 146 *C--H bond moment reference point is 0.3.
Within the above description of preferential plasticizers there are two useful types. These will be designated as volatile and nonvolatile plasticizers. The major practical difference between the two is that the nonvolatile plasticizers remain with the final product while the volatile plasticizers are evolved from the ionomer once they have performed their function.
The nonvolatile plasticizers have, in addition to the above-mentioned properties, a melt point or reversible decomposition point which is substantially above the softening point of the thermoplastic backbone (i.e., at least about 10.degree., and preferably at least about 20.degree.C., above the softening point of the backbone). This melt point should also be in the vicinity of the preselected processing temperature, i.e., at or below the preselected processing temperature.
The nonvolatile preferential plasticizers useful in the instant invention plasticize ionomers only when in the fluid state; they act essentially as fillers at temperatures below their melting points. However, in order for these plasticizers to be useful, they must be readily dispersible in their solid state in the ionomer of interest. It is often helpful, therefore, if the plasticizer contains one or more hydrocarbon moieties. However, it must not be so readily dispersible that it becomes completely molecularly soluble in the ionomer. If this were the case the plasticizer would not retain an independent melt point and would act as a plasticizer at all temperatures.
Typical examples of nonvolatile preferential plasticizers include polar materials containing oxygen, phosphorus or nitrogen atoms. Examples of these plasticizers include compounds such as calcium stearate, zinc laurate, aluminum ricinoleate, lauric acid, benzyl alcohol, resorcinol, distearate ketone, diglycol distearate, dimethylphthalate, nonyl phenol, nonyl phenoxide, triphenylphosphate, tris(3,5-dimethylphenyl) phosphate, diphenylguanidine, piperazine, hydrated salts such as LiSO.sub.4 eH.sub.2 O, etc., alcoholated salts such as CaCl.sub.2 e(CH.sub.3 OH).sub.4, etc.
The second type of preferential plasticizer is the volatile plasticizer. This type of agent relaxes the ionic bonds in the system across the range of temperatures from its melting point to its actual boiling point and allows fabrication of the ionomer to take place across the same temperature range. Except for casting from solutions, the boiling point of the volatile plasticizer under the processing conditions employed should be substantially above that of the softening point of the thermoplastic backbone (i.e., at least about 10.degree., and preferably about 20.degree., above said softening point). Of course, more volatile preferential plasticizers can be employed when solvent casting processes are used.
Typical examples of volatile preferential plasticizers include water and ethers such as tetrahydrofuran; p-dioxane; diethyl ether; butylphenyl ether; alcohols such as methyl alcohol, isopropyl alcohol, and n-butyl alcohol, etc.; phosphorus containing compounds such as tributylphosphate, triisopropylphosphate, etc.; materials containing halogens such as chloroform, bromoform, 1,1,1-trichloroethane, 1-chlorooctane, etc.; materials containing nitrogen such as ethylamine, aniline, dihexylamine, etc.; materials containing sulfur such as 1-butanethio, etc.
Having set forth the types of plasticizers which may be employed in the process of the instant invention, practice of the process of the instant invention will be discussed employing each type of plasticizer with the thermoplastic ionomer. Volatile plasticizers are employed as follows. A sufficient amount of volatile plasticizer is added to the ionomer, i.e., enough to disrupt the ionic domains of said ionomer. Of course this amount will be dependent upon the mole % of polar groups pendent to the thermoplastic backbone and forming ionomeric linkages. However, the amount will normally be in the range of from about 0.1 to about 50, preferably about 0.2 to 20, moles plasticizer per mole of ionic groups. The ionomer containing plasticizer is then heated to a temperature between the softening point of the thermoplastic backbone and the boiling point of the volatile plasticizer. At this temperature the polymer will be in a molten state and may be readily formed in a first desired shape. Thereafter the plasticizer is evolved either by heating the molded article above the boiling point of the plasticizer or by decreasing pressure on the system so as to evolve the plasticizer or by any convenient combination of these techniques. Once the plasticizer has evolved the ionic domains will reform. At this point the formed article may be either cooled below the softening point of the thermoplastic backbone and stored indefinitely or immediately reformed at a temperature above the softening point of the thermoplastic backbone to a second desired shape and then cooled below that softening point while retaining the second desired shape. At any time thereafter the first desired shape may be regained by simply heating the product to a temperature above the softening point of the thermoplastic backbone.
When employing nonvolatile plasticizers, the instant process may be carried out as follows. Sufficient nonvolatile plasticizer having a melting point substantially above the softening point of the thermoplastic backbone is added to the ionomer. By "sufficient" is meant an adequate amount of plasticizer to disrupt the ionic domains of the ionomer. Again, this amount is directly related to the mole % of polar groups pendent to the thermoplastic backbone and forming the ionomeric cross-linkages. Generally, the amount of nonvolatile plasticizer employed is in the range of about 0.1 to about 50, preferably 0.2 to 20, moles per mole of ionic group. Thereafter, the ionomer is heated to a temperature above the melting point of the plasticizer, preferably at least 10.degree.C., and most preferably at least 20.degree.C., above this melting point. At this temperature the plasticized ionomer is in a molten state and may be readily formed in a first desired shape. Thereafter, the formed article is cooled to a temperature between that of the softening point of the thermoplastic backbone and the melting point of the nonvolatile plasticizer. At this temperature the nonvolatile plasticizer solidifies and acts essentially as a filler. It no longer disrupts the ionic domains. The article may then be reformed to a second desired shape and thereafter cooled to a temperature below the softening point of the thermoplastic backbone while retaining the second desired shape. Alternatively, the thermoplastic ionomer after having been formed in the first desired shape may be cooled below both the melting point of the plasticizer and the softening point of the backbone and stored in that manner indefinitely. Thereafter, the article may be reheated to a temperature between the softening point of the backbone and the melting point of the plasticizer and the process continued as shown above.
As in each of the other variations on this process, this article formed to a second desired shape may at any time be heated to a temperature between the softening point of the thermoplastic backbone and the melting point of the plasticizer in order to regain the first desired shape.